> > * Linus Torvalds <torvalds@linux-foundation.org> wrote:> > > On Tue, Aug 3, 2010 at 12:45 PM, Mathieu Desnoyers> > <mathieu.desnoyers@efficios.com> wrote:> > >> > > The real issue here, IMHO, is that Perf has tied gory ring buffer > > > implementation details to the userspace perf ABI, and there is now strong > > > unwillingness from Perf developers to break this ABI.> > (Wrong.)> > > The thing is - I think my outlined buffer fragmentation model would work > > fine with the perf ABI too. Exactly because there is no deep structure, > > just the same "stream of small events" both from a kernel and a user model > > standpoint. Sure, the stream would now contain a new event type, but that's > > trivial. It would still be _entirely_ reasonable to have the actual data in > > the exact same ring buffer, including the whole mmap'ed area.> > Yeah.> > > Of course, when user space actually parses it, user space would have to > > eventually defragment the event by allocating a new area and copying the > > fragments together in the right order, but that's pretty trivial to do. It > > certainly doesn't affect the current mmap'ed interface in the least.> > > > Now, whether the perf people feel they want that kind of functionality, I > > don't know. It's possible that they simply do not want to handle events that > > are complex enough that they would have arbitrary size.> > Looks useful. There's a steady trickle of new events and we already use type > encapsulation for things like trace events - which are only made sense of > later on in user-space.> > We may want to add things like a NOP event to pad out the end of page

/me once again experiences the subtle difference between 'Y' and 'N' when postponing a mail

So adding fragments would be possible as well. We've got the space for such extensions in the ABI and the basic model of streaming information is not affected.

[ The control structure of the mmap area is there for performance/wakeup optimizations (and to allow the kernel to lose information on producer overload, while still giving user-space an idea that we lost data and how much) - it does not affect semantics and does not limit us. ]

So there's no design limitation - Peter simply prefers one possible solution over another and outlined his reasons - we should hash that out based on the technical arguments.